The amount of negative torque generated is usually directly proportional to the back pressure of the engine.
Some innovations increase the exhaust back-pressure by various means, leading to more torque at the flywheel, and therefore more braking power.
More performance can be achieved by down shifting the vehicle (increasing the leverage, or gear ratio of the engine over the wheels).
These circular cuts are made because it is not expected to block the whole cross section of the exhaust pipe.
In this position the butterfly valve remains perpendicular to the flow of exhaust gases and thus creates back pressure on the engine.
In this position the butterfly valve remains parallel to the path of exhaust gases and thus releases the pressure on the engine and allows its speed to increase.
To create sufficient back pressure to reduce the engine speed, it requires a large force from the valve which is made possible by a two-valve arrangement.